Method For Manufacturing Aluminum Alloy Extruded Material

ABSTRACT

The method for manufacturing an aluminum alloy extruded material using an aluminum alloy containing 20 to 95% by mass of a recycled aluminum material made by collecting and remelting extruded materials of aluminum alloys that are used or scrap materials generated in a manufacturing process, containing by mass: 6.0 to 8.0% of Zn, 1.0 to 2.0% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.25% of Zr, and 0.005 to 0.05% of Ti, with 0.30% or less of Si and 0.40% or less of Fe as impurities, and a balance being Al, includes cooling an extruded material at a cooling rate of 50 to 750° C./min from an extruded material temperature of 325 to 550° C. directly after extrusion, and thereafter performing two-stage artificial aging treatment at 90 to 130° C. for 1 to 8 hours and at 130 to 180° C. for 1 to 20 hours.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/JP2022/004678, having an international filing date of Feb. 7, 2022, which designated the United States, the entirety of which is incorporated herein by reference. Japanese Patent Application No. 2021-028184 filed on Feb. 25, 2021 is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present disclosure relates to a method for manufacturing an extruded material using an Al—Zn—Mg-based aluminum alloy, and in particular, can effectively utilize a recycled aluminum material.

In high-strength aluminum alloys that are used for extruded materials, Al—Mg—Si-based 6000 series aluminum alloys, and Al—Zn—Mg-based 7000 series aluminum alloys are mainly known.

From the aluminum alloy used for extruded materials, long billets are continuously cast to be used, by injecting molten metal with an alloy composition in which components are adjusted to predetermined ranges from above to a casting mold and cooling and solidifying the molten metal from a bottom part or below the casting mold.

In this manufacturing process, Si and Fe components are easily mixed in as impurities.

Accordingly, mixing in of Si and Fe becomes a problem, when the used products that were once produced as extruded materials using billets of aluminum alloys, and scrap materials such as remnants generated in the manufacturing process of products are remelted and used as recycled aluminum materials.

In Al—Si—Mg-based aluminum alloys, the content of Si is originally large, and the allowable range of the content of Fe is relatively large, so that mixing in of Si and Fe does not become a big problem, but in Al—Zn—Mg-based aluminum alloys, mixing in of Si and Fe causes reduction in strength, and deterioration of bending formability and the like, and becomes an important problem.

For example, JP-B-2928445 discloses a high-strength aluminum alloy extruded material containing 5.0 to 7.0 wt % of Zn, 1.0 to 1.50 wt % of Mg, 0.1 to 0.3 wt % of Cu, 0.05 to 0.20 wt % of Zr, 0.03 to 0.2 wt % of Cr, 0.3 wt % or less of Mn, and 0.001 to 0.05 wt % of Ti, and a balance including Al and unavoidable impurities.

In the aluminum alloy extruded material disclosed in the same publication, Si and Fe are also dealt as impurities, and looking at the example, Si is suppressed to 0.1 wt % or less, and Fe is suppressed to a level of 0.21 wt % or less.

This is because 0.03 to 0.2 wt % of Cr is added to suppress the size and depth of the recrystallized layer formed on the surface of the extruded material, and therefore it is necessary to suppress the amounts of impurities such as Si and Fe to small amounts in order to secure high strength and SCC (Stress Corrosion Cracking) resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates compositions of aluminum alloys used for evaluation.

FIG. 2 illustrates billet casting and extrusion conditions.

FIG. 3 illustrates evaluation results of extruded materials.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being “connected” or “coupled” to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected or coupled to each other with one or more other intervening elements in between.

The disclosure has an object to provide a method for manufacturing an aluminum alloy extruded material that can increase an allowable range of impurities Si and Fe and obtain high strength so as to enable use of recycled aluminum materials.

A method for manufacturing an aluminum alloy extruded material according to the disclosure is a method for manufacturing an aluminum alloy extruded material using an aluminum alloy containing 20 to 95% by mass of a recycled aluminum material made by collecting and remelting extruded materials of aluminum alloys that are used or scrap materials generated in a manufacturing process, containing by mass: 6.0 to 8.0% of Zn, 1.0 to 2.0% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.25% of Zr, and 0.005 to 0.05% of Ti, with 0.30% or less of Si and 0.40% or less of Fe as impurities, and a balance being Al, including cooling an extruded material at a cooling rate of 50 to 750° C./min from an extruded material temperature of 325 to 550° C. directly after extrusion, and thereafter performing a two-stage artificial aging treatment at 90 to 130° C. for 1 to 8 hours and at 130 to 180° C. for 1 to 20 hours.

In the disclosure, the recycled aluminum material is remelted, and by adding a virgin material to this, component adjustment for the molten metal is performed.

Here, 0.35% or less of Mn may be further contained, and 0.25% or less of Sr may be further contained.

Thereby, an extruded material having a tensile strength equal to or more than 400 MPa, and a 0.2% yield strength equal to or more than 380 MPa is obtained.

The disclosure is capable of quench hardening at the cooling rate of an air-cooling level directly after extruding while increasing the allowable range of the mixing amount of Si and Fe as impurities to by mass: 0.30% or less of Si and 0.40% or less of Fe to enhance the usage ratio of the recycled aluminum material, and the composition of the aluminum alloy that is set to secure SCC resistance with high strength will be described below.

Zn Component

In a 7000 series aluminum alloy, a Zn component has the highest content, since there is little decrease in extrudability even at a relatively high concentration of Zn.

However, when Zn is excessively added, stress corrosion cracking resistance is deteriorated, and therefore, a range of 6.0 to 8.0% by mass of Zn is preferable.

Mg Component

An Mg component is an important additive component along with Zn because high strength is obtained by precipitates of MgZn₂ with the Zn component, but as the addition amount increases, the extrudability deteriorates and bending formability also deteriorates, so that a range of 1.0 to 2.0% by mass of Mg is preferable.

Cu Component

A Cu component improves the strength by solid solution, and has an action of lowering the potential difference with a PF zone by existing together with MgZn₂ in the crystal grain boundary of the metal texture, thereby improving the SCC resistance.

Here, the PF zone refers to regions (Precipitate-Free-Zone) without precipitates observed on both sides of the grain boundary.

However, when Cu is excessively added, extrudability is deteriorated, and general corrosion resistance is deteriorated, so that a range of 0.10 to 0.50% by mass of Cu is preferable.

Zr, Mn, and Cr Components

Zr, Mn and Cr components are all transition elements that have an action of suppressing the depth of the recrystallized layer formed on the surface of the extruded material at the time of extrusion, and an effect of refining crystal grains, and improve the SCC resistance.

However, there are differences in the effects on quench hardening immediately after extrusion, the Cr component makes the quench hardening sensitivity the sharpest, and the required high strength cannot be obtained without high-speed cooling at a water-cooling level in die edge quench hardening.

It is the Mn component that sharpens the quench hardening sensitivity next, and the Zr component is the least sensitive to quench hardening, so that in the disclosure, adjustment is made by addition of Zr and Mn, and the Cr component is reduced as much as possible.

Accordingly, it is preferable that 0.10 to 0.25% by mass of Zr and 0.35% by mass or less of Mn are set, and Cr is not preferably added, and if Cr is added, it is preferable to keep the level of unavoidable impurities below 0.05% by mass.

Sr Component

The Sr component has a great effect on the crystal structure when casting billets, and adding a very small amount of the Sr component suppresses coarsening of crystal grains and suppresses recrystallization on the surface of the extruded material during extrusion.

Although not an essential component in the disclosure, 0.25% by mass or less of Sr is preferably added.

However, when Sr is excessively added, coarse crystallized substances appear and the strength decreases, so that it is necessary to adjust the amount of Sr with the amount of addition of the transition elements, and a total of [Mn+Zr+Sr] is set in a range of 0.25 to 0.50% by mass.

Ti Component

A Ti component is effective in refining crystal grains during billet casting, and Ti is preferably in a range of 0.005 to 0.05% by mass.

Note that a very small amount of B is often contained.

Next, casting and homogenization treatment of a billet will be described.

A billet for extrusion is generally continuously cast as a long cylindrical billet.

As a casting method, various methods such as a hot top casting method and a float type casting method are performed, and in either case, the aluminum alloy is casted into a long cylindrical billet by being cooled from a periphery at a bottom part of a casting mold or a lower side of the casting mold and being solidified.

The billet used in the disclosure preferably has a casting structure of a fine structure composed of fine crystal grains, and a casting rate at which it is cooled and solidified, and cast on the lower side of the casting mold is preferably 50 mm/min or higher, and as a result, the fine structure of the billet preferably becomes a casting structure with an average grain size of 250 μm or less, more preferably 200 μm or less.

The extrusion conditions will be described.

An extruder has a container with an extrusion die attached to a front side, a cylindrical billet is loaded into the container, and is hot-extruded from behind by a stem or the like.

Here, the billet is loaded into the container in a state in which the billet is preheated to 400° C. or higher, preferably 430 to 510° C., and extruded.

The extruded material extruded by hot working also has a high temperature due to heat of working, but it is preferable to secure 440° C. or higher in order to sufficiently perform subsequent quench hardening, and at least 325° C. or higher is required at a time of start of cooling by air cooling.

Further, when the temperature of the extruded material directly after extrusion exceeds 550° C., it is not preferable because gouge defects tend to occur in an appearance.

For the extruded material extruded as described above, die edge quench hardening by air cooling is performed.

The cooling rate in a range of 50 to 750° C./min is secured by fan air cooling or the like.

In conventional water cooling, the extruded material has been often locally cooled rapidly, and strain deformation such as cross-sectional deformation has easily occurred in the extruded material, whereas in the disclosure, high strength is sufficiently obtained by air cooling, and cooling strain deformation can also be suppressed.

The artificial aging treatment after extrusion will be described.

An extruded material made of a 7000 series aluminum alloy can obtain high strength by precipitating G. P. zones and intermediate phases in the crystal structure of the extruded material, and a two-stage artificial aging treatment is performed at 90 to 130° C. for 1 to 8 hours for a first stage, and at 130 to 180° C. for 1 to 20 hours for a second stage.

In the disclosure, by adopting the aluminum alloy composition and manufacturing conditions as described above, the usage amount of recycled material can be increased, and extruded materials with high strength and excellent in SCC resistance can be obtained.

Exemplary embodiments are described below. Note that the following exemplary embodiments do not in any way limit the scope of the content defined by the claims laid out herein. Note also that all of the elements described in the present embodiment should not necessarily be taken as essential elements

Compositions of various aluminum alloys were adjusted, and cylindrical billets with a diameter of 8 inches were experimentally produced and evaluated while examining extrusion conditions, as will be described below.

Long billets of 8 inches were cast at casting rates shown in a table of FIG. 2 by using molten metals of aluminum alloys having various alloy compositions shown in a table of FIG. 1 .

In this case, in order to adjust the compositions in a table of FIG. 1 , recycled aluminum materials in proportions shown in a table of FIG. 2 were used.

Next, directly after homogenization treatment was performed under homogenization treatment conditions shown as “HOMO” in the table of FIG. 2 , cooling was carried out under conditions of “cooling rate after HOMO” in the table of FIG. 2 .

Note that respective conditions shown in the table show conditions suitable for the disclosure.

In the table, “billet crystal grain size” was obtained by measuring a value of an average crystal grain size by an optical microscope after cutting out a test piece of a cast cross-section from a billet, and applying polishing and etching to the test piece.

In the table, “BLT temperature” indicates a preheating temperature when the billet is loaded into a container of an extruder, “profile temperature after extrusion” indicates a surface temperature of an extruded material directly after extrusion, “profile temperature at the start of cooling” and “cooling rate after extrusion” indicate a surface temperature of the extruded material at a start of die edge quench hardening and a cooling rate by fan air cooling.

In the table, “heat treatment conditions” indicate the conditions and a treatment time period of artificial aging treatment.

Evaluation results are shown in a table of FIG. 3 .

“T5 tensile strength”, “T5 yield strength”, and “T5 elongation” in the table were measured by cutting out JIS-Z2241 and JIS-5 test pieces in an extruding direction from the extruded materials subjected to the two-stage artificial aging treatment by a tensile tester conforming to JIS standards.

As for “SCC” in the table, presence or absence of cracks was evaluated by cutting out the extruded material into test pieces in the extruding direction, applying 80% of stress of a 0.2% yield strength value shown in “T5 yield strength” in the table in a bending direction, and carrying out 720 cycles under the following conditions.

In the table, “circle” indicates that no crack occurred.

SCC Resistance Test One Cycle

After the test pieces were immersed into a 3.5% NaCl aqueous solution at 25° C., for 10 minutes, the test pieces were held in an atmosphere of 25° C. and 40% humidity for 50 minutes, and thereafter taken out from a test furnace to dry naturally.

As for “microstructure, surface recrystallization depth” in the table, an extruded cross-section of the extruded material was polished and etched, and the depth of the recrystallized layer formed on a surface side of the extruded material was measured by an optical microscope.

From the evaluation results in FIG. 3 , the following can be said.

In each of examples 1 to 8, the composition of the aluminum alloy, the casting conditions of the aluminum billet, and the extrusion conditions were within the set ranges, so that the usage rate of the recycled aluminum material was able to be set high within the range of 20 to 95%.

In contrast, in comparative example 1, the recycled aluminum material was 100%, so that the Si content was not able to be suppressed to 0.30% or less, the Fe content was not able to be suppressed to 0.40% or less, and the SCC resistance did not reach the target.

In comparative example 2, the usage amount of the recycled aluminum material was suppressed to 25%, but the alloy composition was outside the setting, so that the SCC resistance did not reach the target.

In comparative example 3, the recycled aluminum material that was used accounted for 100%, and in this case, the strength did not reach the target, either.

In the disclosure, aluminum alloy extruded materials having a high strength and excellent in SCC resistance can be obtained while recycled aluminum materials are effectively utilized, and the aluminum alloy extruded materials can be used for structure members of vehicles and various machines. 

What is claimed is:
 1. A method for manufacturing an aluminum alloy extruded material using an aluminum alloy containing 20 to 95% by mass of recycled aluminum material made by collecting and remelting extruded materials of aluminum alloys that are used or scrap materials generated in a manufacturing process, containing by mass: 6.0 to 8.0% of Zn, 1.0 to 2.0% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.25% of Zr, and 0.005 to 0.05% of Ti, with 0.30% or less of Si and 0.40% or less of Fe as impurities, and a balance being Al, comprising: cooling an extruded material at a cooling rate of 50 to 750° C./min from an extruded material temperature of 325 to 550° C. directly after extrusion; and thereafter performing two-stage artificial aging treatment at 90 to 130° C. for 1 to 8 hours and at 130 to 180° C. for 1 to 20 hours.
 2. The method for manufacturing an aluminum alloy extruded material according to claim 1, wherein the aluminum alloy further containing 0.35% or less of Mn and containing 0.25% or less of Sr is used.
 3. The method for manufacturing an aluminum alloy extruded material according to claim 1, wherein a tensile strength is equal to or more than 400 MPa, and a 0.2% yield strength is equal to or more than 380 MPa.
 4. The method for manufacturing an aluminum alloy extruded material according to claim 2, wherein a tensile strength is equal to or more than 400 MPa, and a 0.2% yield strength is equal to or more than 380 MPa. 